Have you ever wondered how many water molecules it takes to form the crystal beginnings of every ice cube and snowflake? Until recently researchers weren’t quite sure. But we now know the answer: 275.

But how did they get such a precise number? Water absorbs light at one frequency, ice crystals at another. By using a few tricks to control the number of molecules, the scientists examined the response of tiny clusters of water molecules to infrared light at sub-freezing temperatures—and slowly increased the number of molecules until they saw a change. When the number of molecules reached 275—bingo—the clusters began absorbing light like ice crystals. At 475 molecules the crystal is completely formed. The study is in the journal Science. [Christoph C. Pradzynski et al., A Fully Size-Resolved Perspective on the Crystallization of Water Clusters]

The research creates the first-ever picture of how ice crystals are gradually formed from water. Researchers say their method can be adapted for use with many other types of molecules and may potentially be used to look at a variety of chemical reactions between the interfaces of liquids, gases or solids—at the individual molecular level.